Japanese Patent Publication (unexamined) No. 89296/1978 discloses an apparatus for determining the motion of a jaw wherein the motion of the lower jaw is determined by a photosensor sensing light from a light source attached to the lower jaw. The photosensor of the apparatus is composed of light-receiving sensors, such as CCDs and phototransistors, which sense light from the light source. Accordingly, in order to enhance accuracy, it is required that the light-receiving sensors increase in number, but which makes the apparatus more expensive. Also, a calculation process and calculation circuit become complicated, since the direction of the light is changed to accompany the position change of the lower jaw. Further, in the apparatus mentioned above, when the head is moved together with the lower jaw, the motion of the head is also detected as the motion of the lower jaw. It, therefore, is required to fix the head, because the motion of the head causes determination errors. However, when one fully opens one's mouth, one has to raise the head up a little. Accordingly, it is difficult for the apparatus to carry out an accurate determination.
Japanese Utility Model Publication (unexamined) No. 34290/1979 discloses an apparatus which determines the motion of the lower jaw with three potentiometers. This apparatus has a complicated mechanism for transmitting the motion of the jaw to the potentiometers. Also a driving mechanism for the potentiometer has to have a play which causes determination errors. Further, since power is required to drive the potentiometers, it interferes with a natural motion of the jaw.